Differential temperature and pH controls on the abundance and composition of H-GDGTs in terrestrial hot springs

H-shaped glycerol dialkyl glycerol tetraethers (H-GDGTs), also called glycerol monoalkyl glycerol tetraethers (GMGTs), are a unique group of membrane lipids found in several lineages of Euryarchaeota and Crenarchaeota. Their function in these taxa is, however, not well understood. Here we show their presence in both core lipid and polar lipid fractions from microbial biomass sampled from hot springs in Tibet, China (21.9-80.0 °C; pH 7.0-9.1), Tengchong, China (55.1-93.6 °C; pH 2.5-9.4) and Yellowstone National Park (YNP), USA (16.3-86.7 °C; pH 2.1-9.6) using high performance liquid chromatography-mass spectrometry. The number of cyclopentyl rings ranged from zero (H-GDGT-0) to six (H-GDGT-6) in lipid fractions in Tengchong and YNP and from zero to four (H-GDGT-4) in those from Tibet. While H-GDGT-0 was the most abundant H-GDGT in Tibetan springs, H-GDGT-4 predominated in Tengchong and H-GDGT-6 predominant in YNP, resulting in higher ring indices in the latter springs. While pH appeared to be the most important factor affecting the variation in the relative abundance and average number of cyclopentane rings in H-GDGTs from YNP communities, both temperature and pH appeared to be important controls on the abundance of H-GDGT lipids in Tengchong communities, whereas neither pH nor temperature appeared to influence the abundance or average number of cyclopentane rings in H-GDGTs from Tibetan spring communities. Furthermore, the relative abundance of H-GDGTs to total iGDGTs was greater in hot springs with acidic pH, particularly those from YNP. This finding, coupled with taxonomic profiling of archaeal 16S rRNA genes recovered from the same YNP springs, indicates that H-GDGTs in acidic springs may be synthesized by members of the archaeal order Thermoplasmatales, which are adapted to acidic pH.